Dilution apparatus for a thickener

ABSTRACT

The invention provides an axial flow pump ( 1 ) for accurately injecting supernatant liquid ( 2 ) from adjacent the liquid surface ( 3 ) of a thickening tank (not shown) to dilute liquid in a feedwell ( 4 ). The pump ( 1 ) includes a cup shaped receiving vessel ( 5 ) submerged in the supernatant liquid. The pump also includes a submerged impeller ( 6 ) mounted on a rotatable shaft ( 7 ) for drawing liquid from the vessel ( 5 ) and injecting it into the feedwell ( 4 ). The shaft ( 7 ) is driven by a motor and associated gearbox (not shown). The pump is responsive to system parameters for adjusting the flow rate of the supernatant liquid.

FIELD OF THE INVENTION

The present invention relates to separation devices for liquid suspensions and pulps and in particular to recycling of overflow liquor in thickeners. It has been developed primarily for use in separating mineral pulp from a liquid suspension and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND OF THE INVENTION

The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an admission that such art is widely known or forms part of common general knowledge in the field.

Thickeners are known as a means of separating suspended pulps. They are commonly used in the field of mineral processing and extraction, as well as in other industries.

Known thickeners typically include a thickening tank and a feedwell. The feedwell includes a chamber having an inlet for receiving feed slurry and an outlet in fluid communication with the tank. In use, flocculation takes place in the tank, whereby pulp of higher relative density tends to settle towards the bottom of the tank, forming a bed of thickened pulp, and dilute liquor of lower relative density is thereby displaced towards the top of the tank.

The feedwell is configured to reduce the turbulence of the incoming feed slurry, to allow flocculants to be mixed into the liquid, and to allow sufficient residence time for reaction between flocculants and reagents, before the treated liquid is discharged into the thicken tank. The configuration of the feedwell is typically also intended to promote uniform distribution of slurry flowing from its outlet into the thickening tank.

Feed slurry concentration affects the rate of floc formation, and the dosage rate of flocculant required to achieve optimum separation. It also affects thickener performance in terms of overflow clarity and underflow density versus flocculant consumption. Accordingly, the feed slurry entering the feedwell frequently requires dilution in order to achieve an optimum result.

Existing technologies achieve feed slurry dilution by several methods. Some methods involve pumping supernatant or new liquid into the feed stream. Relatively newer methods, however, use liquid from within the thickening tank to dilute the feed stream. These newer methods typically employ mechanisms of differential density or momentum transfer to move the liquid from the thickening tank into the feed stream.

All of these existing methods have disadvantages. Those utilising pumps require considerable pipework and electrical power to transfer the diluting liquid. Conversely, the internal systems, which use differential density or momentum transfer mechanisms, are relatively simpler and require little to no electrical power, but lack means to precisely control dilution under all operating conditions.

It is an object of the invention to overcome or substantially ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.

DISCLOSURE OF THE INVENTION

According to a first aspect, the invention provides a dilution apparatus for a thickener of the type having a thickening tank and a feedwell disposed upstream of the tank to receive feed liquid, said dilution apparatus including:

-   -   a submersible pump adapted for at least partial submersion         within the thickening tank, for withdrawing supernatant dilution         liquid directly from the thickening tank and injecting said         dilution liquid into the feed liquid, to effect dilution thereof         before transfer of the feed liquid from the feedwell into the         thickening tank.

Preferably, the apparatus further includes control means to regulate the flow rate of the dilution liquid in response to one or more selected system parameters. The flow rate is preferably varied by adjusting the pump speed.

In the preferred embodiment, the feedwell is disposed substantially within the thickening tank, the pump is positioned substantially within the tank, and all plumbing and pipe work for the pump is predominantly contained within an outer perimeter of the tank. Preferably also, the pump is positioned closely adjacent the feedwell so as to minimise intermediate plumbing and pipe work within the tank.

The pump is preferably an axial flow pump, including a generally cup shaped receiving vessel adapted, in use, to be submerged in the supernatant dilution liquid. The pump preferably includes an impeller mounted on a rotatable shaft for drawing liquid from the receiving vessel and injecting it into the feedwell. The shaft is preferably driven by a motor and gearbox.

Preferably also, the impeller and shaft are housed in a generally vertically oriented elongate chamber extending upwardly from below the impeller. The chamber is preferably cylindrical in shape, and includes an outlet port for directing pumped fluid into the feedwell.

In one preferred embodiment, the dilution liquid is injected into a feed pipe upstream of the feedwell. In an alternative embodiment, however, the dilution liquid is injected directly into the feedwell.

According to a second aspect, the invention provides a thickener including:

-   -   a thickening tank;     -   a feedwell located upstream of said tank to receive feed liquid;         and     -   a dilution apparatus as previously defined;     -   the submersible pump being at least partially submerged within         the thickening tank, for withdrawing supernatant dilution liquid         directly from the thickening tank and injecting said dilution         liquid into the feed liquid to effect dilution thereof, before         transfer of the feed liquid to the tank.

Preferably, the thickener further includes control means to regulate the flow rate of the dilution liquid in response to one or more selected system parameters.

In the preferred embodiment, the feedwell is disposed substantially within the thickening tank, the pump is positioned substantially within the tank, and all plumbing and pipe work for the pump is substantially contained within the outer perimeter of the tank. Preferably also, the pump is positioned closely adjacent the feedwell so as to minimise intermediate plumbing and pipe work within the tank. The feedwell is preferably positioned substantially centrally within the tank.

According to a third aspect, the invention provides a method of diluting feed liquid in a thickener having a thickening tank and a feedwell located upstream of said tank to receive feed liquid, said method including the steps of:

-   -   providing a submersible pump;     -   submerging the pump at least partially within the thickening         tank;     -   actuating the pump so as to withdraw supernatant dilution liquid         directly from the thickening tank, and injecting said dilution         liquid into the feed liquid to effect dilution thereof, before         transferring to the feed liquid from the feedwell to the tank.

Preferably, the method includes the further step of controlling the flow rate of the dilution liquid in response to one or more selected system parameters.

Preferably, the method also includes the step of maintaining a relatively small head difference between a liquid surface in the feedwell and a liquid surface in the thickening tank.

In the preferred embodiment, the method includes the steps of positioning the feedwell substantially within the thickening tank, positioning the pump substantially within the tank, and containing the plumbing and pipe work for the pump substantially within the outer perimeter of the tank. Preferably also, the pump is positioning closely adjacent the feedwell so as to minimise the intermediate plumbing and pipe work within the tank.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a sectional side elevation of dilution apparatus according to the invention.

FIG. 2 is a plan view of a thickener feedwell incorporating the dilution apparatus.

PREFERRED EMBODIMENTS OF THE INVENTION

A preferred application of the invention is in the field of mineral processing, separation and extraction, whereby finely ground ore is suspended as pulp in a suitable liquid medium, such as water, at a consistency which permits flow, and settlement in quiescent conditions.

Referring to the drawings, the apparatus includes an axial flow pump 1 for accurately injecting supernatant liquid 2 from adjacent the liquid surface 3 of a thickening tank (not shown) to dilute liquid in a feedwell 4.

The pump 1 includes a cup shaped receiving vessel 5 submerged in the supernatant liquid. The pump also includes a submerged impeller 6 mounted on a rotatable shaft 7 for drawing liquid from the vessel 5 and injecting it into the feedwell 4. The shaft 7 is driven by a motor and associated gearbox (not shown). The pump is responsive to system parameters for adjusting the flow rate of the supernatant liquid. In the illustrated embodiment, the flow rate is adjusted by varying the pump speed. However, in other embodiments, the flow rate is adjusted using control valves or other means.

The impeller 6 and shaft 7 are housed in a vertical cylindrical chamber 8 extending upwards from below the impeller. The chamber extends above the surface 3 of the supernatant liquid, thereby overcoming the requirement for a seal at its apex and allowing for access and removal of the impeller 6. The chamber also includes an outlet port 9 located above the impeller 6 for directing pumped fluid into a feed pipe 10 of the feedwell 4.

In use, the motor is activated to drive the shaft 7 and the impeller 6. The rotation of the impeller draws supernatant liquid from the thickening tank into the vessel 5 and up through the chamber 8. This increases the pressure head between the liquid level in the thickening tank and the outlet port 9 and forces the liquid through the outlet and into the feed pipe 10.

In an alternative embodiment, the vessel 5 includes an additional aperture at its base to provide an additional path for liquid to be drawn.

The dilution apparatus also includes a mounting flange 11 to permit its bolting onto a mounting hub 12 of the feedwell. In other embodiments, the apparatus is bolted onto the side of the feedwell itself to allow supernatant liquid to be injected directly into the feedwell.

In the preferred embodiment described above, the pump 1 is an axial flow pump. However, it will be appreciated that alternative pump types can also be used to inject the supernatant liquid 2 into the feedwell 4. Piston pumps and vacuum pumps are examples of some alternatives pump types that could be used.

It will be appreciated that the illustrated apparatus provides an efficient and accurately controllable means for diluting the feed slurry in a thickener. As the head difference between the liquid surface in the feedwell 6 and the liquid surface 3 in the thickening tank are maintained at a similar level, the axial pump 1 requires only a small pressure head to drive the diluting supernatant liquid. The proximity of the apparatus to the feedwell reduces the amount of piping required and also reduces power consumption. Also, the apparatus does not require a wear resistant impeller as it pumps relatively clean liquid. The apparatus also allows flocculants to be introduced either upstream or downstream of the impeller to achieve dispersion and dilution of flocculants prior to contact with solid particles in the stream. In this embodiment, flocculent is added through apertures 13 in the feed pipe 10.

In all these respects, the invention represents a practical and commercially significant improvement over the prior art.

Although the invention has been described with reference to a specific example, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 

1. A dilution apparatus for a thickener of the type having a thickening tank and a feedwell disposed upstream of the tank to receive feed liquid, said dilution apparatus including: a submersible pump adapted for at least partial submersion within the thickening tank, for withdrawing supernatant dilution liquid directly from the thickening tank and injecting said dilution liquid into the feed liquid, to effect dilution thereof before transfer of the feed liquid from the feedwell into the thickening tank.
 2. A dilution apparatus according to claim 1, further including control means to regulate a flow rate of the dilution liquid in response to one or more selected system parameters.
 3. A dilution apparatus according to claim 2, wherein the flow rate is varied by adjusting the pump speed.
 4. A dilution apparatus according to claim 1, wherein the feedwell is disposed substantially within the thickening tank, wherein the pump is positioned substantially within the tank, and wherein all plumbing and pipe work for the pump is substantially contained within an outer perimeter of the tank.
 5. A dilution apparatus according to claim 4, wherein the pump is positioned closely adjacent the feedwell so as to minimise intermediate plumbing and pipe work within the tank.
 6. A dilution apparatus according to claim 1, wherein the pump is an axial flow pump.
 7. A dilution apparatus according to claim 1, wherein the pump includes a generally cup shaped receiving vessel adapted, in use, to be submerged in the supernatant dilution liquid.
 8. A dilution apparatus according to claim 7, wherein the pump includes an impeller mounted on a rotatable shaft for drawing liquid from the receiving vessel and injecting it into the feedwell.
 9. A dilution apparatus according to claim 8, wherein the shaft is driven by a motor and gearbox.
 10. A dilution apparatus according to claim 8, wherein the impeller and shaft are housed in a generally vertically oriented elongate chamber extending upwardly from below the impeller.
 11. A dilution apparatus according to claim 10, wherein the chamber is generally cylindrical in shape, and includes an outlet port for directing pumped fluid into the feedwell.
 12. A dilution apparatus according to claim 1, wherein the dilution liquid is injected into a feed pipe upstream of the feedwell.
 13. A dilution apparatus according to claim 1, wherein the dilution liquid is injected directly into the feedwell.
 14. A thickener including: a thickening tank; a feedwell located upstream of said tank to receive feed liquid; and a dilution apparatus as defined in any one of the preceding claims; the submersible pump being at least partially submerged within the thickening tank, for withdrawing supernatant dilution liquid directly from the thickening tank and injecting said dilution liquid into the feed liquid to effect dilution thereof, before transfer of the feed liquid to the tank.
 15. A thickener according to claim 14, further including control means to regulate a flow rate of the dilution liquid in response to said one or more selected system parameters.
 16. A thickener according to claim 14, wherein the feedwell is disposed substantially within the thickening tank, wherein the pump is positioned substantially within the tank, and wherein all plumbing and pipe work for the pump is substantially contained within an outer perimeter of the tank.
 17. A thickener according to claim 16, wherein the pump is positioned closely adjacent the feedwell so as to minimise intermediate plumbing and pipe work within the tank.
 18. A thickener according to claim 16, wherein the feedwell is positioned substantially centrally within the tank.
 19. A method of diluting feed liquid in a thickener having a thickening tank and a feedwell located upstream of said tank to receive feed liquid, said method including the steps of, providing a submersible pump; submerging the pump at least partially within the thickening tank; actuating the pump so as to withdraw supernatant dilution liquid directly from the thickening tank, and injecting said dilution liquid into the feed liquid to effect dilution thereof, before transferring to the feed liquid from the feedwell to the tank.
 20. A method according to claim 19, including the further step of controlling the flow rate of the dilution liquid in response to one or more selected system parameters.
 21. A method according to claim 19, wherein the pump is an axial flow type pump.
 22. A method according to claim 19, wherein the pump includes a generally cup shaped receiving vessel submerged in the dilution liquid.
 23. A method according to claim 19, including the further step of drawing liquid from the receiving vessel and injecting it into the feedwell.
 24. A method according to claim 19, wherein the dilution liquid is injected into a feed pipe of the feedwell.
 25. A method according to claim 19, wherein the dilution liquid is injected directly into the feedwell.
 26. A method according to claim 19, including the further step of maintaining a relatively small head difference between a liquid surface in the feedwell and a liquid surface in the thickening tank.
 27. A method according to claim 19, including the further steps of positioning the feedwell substantially within the thickening tank, positioning the pump substantially within the tank, and containing all plumbing and pipe work for the pump substantially within an outer perimeter of the tank.
 28. A method according to claim 27, including the further step of positioning the pump closely adjacent the feedwell and thereby minimising intermediate plumbing and pipe work within the tank.
 29. A method according to claim 28 or claim 29, wherein the feedwell is positioned substantially centrally within the tank. 